LTE is being evolved by 3GPP to be able to handle Mission Critical services for Public Safety operations. One of the new types of service that needs to be handled is the ability for Group communications (and in particular Push To Talk groups) to be operated using LTE networks. Push To Talk communication has low activity by nature, so in order to maximise efficiency of the LTE radio resources, statistical multiplexing of multiple Group Calls can be used to reduce overall radio resource reservation required to handle the Group Call traffic, which in turn allows operator to free up capacity for their consumer broadband and other services. In addition Group Call communications are required to be reliable. For example, police and firemen need to be able to communicate with a high level of reliability.
In addition to this, the nature of Group Calls being Point-To-Multipoint (PTM) means that multiple terminals from the same “talk group” would receive the same data from the same source (data originating from another terminal in the same talk group). A feature called evolved Multimedia Broadcast and Multicast Service (eMBMS) within LTE allows a single data burst to be mapped to the radio resource and transmitted once in a broadcast fashion, such that the same transmitted signal (containing the data) can be received simultaneously by multiple receiving terminals in downlink direction, in order to further optimise the radio resources used.
Another optimisation of eMBMS is that multiple adjacently deployed eNode Bs (deployed to provide a contiguous coverage area) can be synchronised in time, and the eMBMS physical signals can be transmitted or aligned in each eNode B such that a terminal can receive the same signal from multiple eNode Bs nearly simultaneously. This approach is called Single Frequency Network (SFN). The SFN approach both avoids interference at the receiving terminal and increases its Signal-to-Noise Ratio (SNR) of the received eMBMS physical channel. The area over which the same physical signal is sent from multiple eNode Bs simultaneously is called the MBSFN area. In order to ensure that all eNode Bs in an MBSFN area can be coordinated properly, there is a logical node controlling all of them called an MCE.
Given that the usefulness of eMBMS usage depends on a number of terminals all desiring to receive the same data at the same time, a procedure allows the MCE and eNodeB to “count” the number of users interested in the service, and this procedure can be performed periodically. If there are insufficient numbers of terminals interested compared to the radio resource that is being expended in using the eMBMS radio resource, then the users may be switched to “unicast” operation, using a dedicated (point-to-point) channel for each terminal individually within a cell. The mapping process is not a very dynamic process, however, and there is some delay in mapping Group Calls to MBMS physical signals for MBMS reception and switching them back to dedicated channels for unicast operation. The control of this is currently performed by a control channel (at the “RRC” protocol layer) that is sent from the MCE every five seconds to all terminals receiving the MBMS physical signal.
Therefore, the usage of an eMBMS physical signal transmitted in an SFN fashion from multiple eNode Bs in an MBSFN area is an important feature for handling Group Calls. However, given that MBMS resources cannot be reserved with a very fine granularity within a cell and the fact that the level of MBMS resources reserved in the cell cannot be adapted very quickly, multiple Group Calls need to be able to be multiplexed and mapped to the same eMBMS physical signal with the aim to use the reserved resource as effectively as possible. Furthermore, given the low activity factor of Group Calls, and the lack of dynamicity in mapping and de-mapping Group calls to/from MBMS, this leads to a need to rely heavily on statistical multiplexing to estimate the overall MBMS radio resources required.
Furthermore, there is no equivalent of an eMBMS operation in uplink direction, Group Calls transmitting voice or video data would need to use the dedicated (point-to-point) channel in uplink, even if they are receiving voice or video data via the eMBMS physical signals in downlink.
The following text is extracted from 3GPP TS23.468 and 3GPP TS36.440 for further description of the eMBMS architecture for Group Call operation.
FIG. 1 (FIG. 4.2.2-1 from 3GPP TS23.468) shows non-roaming architecture model for GCSE_LTE.
FIG. 2 (FIG. 4-1 from 3GPP TS23.468) shows a simplified architecture for MBMS in LTE/SAE. It consists of EPC functional entities and E-UTRAN nodes. Functions of MBMS EPC entities are defined in TS 23.246. Functions of MBMS E-UTRAN nodes are defined in TS 36.300. It should be noted that TS 36.300 also allows MCE be deployed inside eNode B.
FIG. 2 shows interfaces related to E-UTRAN (i.e. M1, M2 and M3). For MBMS, control signalling and user plane data packet are distributed from the EPC to E-UTRAN through different interfaces.
Control Plane Interfaces:
M3, M2 interface are pure control plane interfaces.
M3 between MME and MCE mainly carries MBMS session management signalling.
A MCE is connected to one or more than one eNode Bs (eNBs) within the same MBSFN through M2 interface mainly for MBMS session management signalling and radio configuration signalling.
User Plane Interface:
M1 interface is a pure user plane interface.
A MBMS GW is connected to multiple eNBs through M1 interface for data distribution.
Reference points within EPC are not in the scope of this document. Please refer to TS 23.246 for details.
Multiple talk groups are typically mapped to MBMS resources in downlink direction and there is the possibility that at any given moment the traffic generated for the active talk groups mapped to the MBMS resource might exceed the capacity of that resource. This is due to the lack of dynamicity of adapting the reserved MBMS resources in the cell, and the lack of dynamicity in switching terminals between unicast and MBMS operation in the cell, and the lack of predictability and accuracy of statistical multiplexing, and the need of LTE operators to try to use their allocated spectrum efficiently. If there are no other MBMS resource “units” available, this might lead to data for one or more “talk group” being discarded by the eNode B prior to transmission over the radio interface, thus not meeting the needs of the service.
Even though MBMS is primarily targeted for talk groups with medium to high numbers of users/terminals within the cell or MBSFN area, the traffic levels generated for these groups may not be typically enough to “fill up” the MBMS resource reserved in the cell, and the lack of granularity of the MBMS resource units reserved means that some of the cell capacity could be wasted. Therefore, a reasonable radio resource management (RRM) mechanism should use the reserved resources more effectively. To enable this, talk groups with low numbers of users in the cell and/or MBSFN area should also be configured to use the MBMS resources that have been reserved even though these groups could also be handled via unicast operation. These talk groups would also be contributing to the traffic loading on the MBMS resource.
Therefore, there is required a method and system that improves the management of the network and the utilisation of communications resources.